In U.S. Pat. No. 3,573,438, there is disclosed a controlled shift register comprising a plurality of parallel connected shift modules, with each module including a thermoresistive element characterized by a metal-semiconductor phase transition, that is, a transition temperature below which the thermoresistive material has semiconductor conduction characteristics and above which the thermoresistive material has metallic conduction characteristics. A heating resistor is connected in series with each thermoresistive element with each heating resistor being disposed adjacent to both the thermoresistive element in series therewith and the thermoresistive element of the next succeeding shift module. The heating resistors of alternate module are connected to oppositely poled control elements. When a shift module is triggered by applying an initial heat pulse to its associated thermoresistive element, the thermoresistive element switches to its low resistance state, and, in turn, allows substantial current to flow through its series connected heating resistor. Due to the adjacent disposition of the heating resistor with its associated thermoresistive element and the thermoresistive element of the next succeeding module, the current flow through the heating resistor continues the heating of its associated thermoresistive element when the initial triggering heat pulse is removed and also initiates heating of the thermoresistive element of the next succeeding shift module and switches it to its low resistance state. Heating of both thermoresistive elements continues until the control element in series with the thermoresistive element of the next succeeding shift module is phased to conduction at which time the heating element in series therewith continues the heating of its series-connected thermoresistive element and initiates heating of the thermoresistive element of the next succeeding module. In this manner, the initial trigger pulse is made to propagate from one shift module to the next shift module as the phase of the voltage across the control elements of the shift register changes.
The referenced shift register offers limited utilization because the thermoresistive element of any stage or module must be continuously heated if it is to be maintained in its low resistance state. As noted, this continuous heating is achieved by disposing each heating resistor adjacent to both the thermoresistive element of its module and the thermoresistive element of the next succeeding module. Thus, the referenced shift register cannot be used to control remote electrical elements, such as a remote line buffer. Also, continuous heating by the heating resistors may cause severe heat sinking problems.